Evaporation of a fuel, which contains a less volatile blend component (i.e. heavier than the fuel molecules or more polar), leads to an uneven distribution of fuel components, whereby the less volatile components tend to enrich in the non-evaporated residue. In combustion engines, the combustion itself will convert most of the fuel material; however, a small amount of the remaining liquid will remain non-combusted in the combustion chamber or on the wall. This is then washed out by and accumulated in the lubricating engine oil. The effect of this fuel dilution is enhanced when the piston rings are worn or defective, since more fuel might directly be pushed past the piston rings into the lubricating engine oil.
So-called “biodiesel” fuel components such as a vegetable oil or vegetable oil derivative (e.g. a fatty acid ester, in particular a fatty acid methyl ester (FAME)), or another oxygenate such as an acid, ketone or ester, are usually less volatile than hydrocarbons usually present in mineral oil derived fuels. The mandatory presence of such renewable fuel components, in particular FAME, has led to an increase of the presence of such components in the lubricating engine oil. This, however, leads to a more rapid breakdown of the lubricating engine oil.
The effects of the enrichment of fuel components in the engine lubricating oil include viscosity reduction due to fuel dilution; sludge and varnish formation; depletion of the alkalinity reserve in the lubricating base oil, as indicated by total base number decrease; a more rapid increase in total acid number, indicating oil degradation due to more nitric acid formation; and leaching of certain metals such as copper and lead from bearings due to the higher solvency of the lubricant base oil comprising the biodiesel component. Other effects are related to the presence of exhaust gas turbochargers as applied in modern diesel engines. Here the biodiesel components carbonise under the elevated temperatures occurring in the turbocharger bearings, resulting eventually in failing bearings and/or seals.
This is further enhanced by the extended oil drain intervals usually applied. In the worst case, even a catastrophic failure of the engine lubricating oil may occur due to gelation or plugging of oil filters from excessive sludge formation. As a result, the oil drain intervals would need to be shortened to avoid lubricant contamination likely to produce engine damage.